Metal plates, screws, wires, and bars have been used since the nineteenth century to immobilize bone fragments and correct various types of bone disorders, but only in recent times has the practice become widespread without the serious risks of infection, tissue rejection, and technical failures observed in early procedures. Today utilization of metal support elements are used routinely for the surgical treatment of bone fractures, corrections, and reconstructions.
Although filled, reinforced hosing (Soviet Union Pat. No. 1745231) and rigid polymers have been suggested as alternatives, metal rods or bars are particularly useful for the surgical correction of spinal column disorders such as scoliosis, kyphosis, spondyloisthesis, and remediation of other problems such as ruptured or slipped discs, broken or fractured spinal columns and the like. Stainless steel is often employed (see, for example, U.S. Pat. No. 4,773,402 to Asher and Strippgen, column 3, lines 51 to 53); it is typically chemically polished and passivated so that it resists corrosion by body fluids. A number of systems have been described, variously utilizing hooks, wires, plates, rods, and combinations of these elements with each other and with pins, screws, and other securing means (see, for example, Dickson, R. A., and Bradford, D. S., eds., Management of Spinal Deformities, Butterworths, London, 1984, pages 194 to 204). Several methods for contouring these elements have been suggested.
In U.S. Pat. No. 4,653,481, Howland and Wiltse disclosed a spine fixation system with a plurality of screw clamp assemblies that included removable saddle assemblies having apertures proportioned for the reception of rigid support rods. In the installation of the system, screw members were positioned in the spine, and a soft rod, e.g., one made of a soft aluminum alloy, was installed through the apertures, forming a temporary master that could be used to replicate the relative position of the apertures (column 10, line 63 to 66). The soft rod was removed and used as a master for shaping the rigid rods (column 11, lines 61 to 68). In a later patent, Howland employed soft aluminum as a master for a steel rod in a functionally similar spine fixation system that had an improved saddle assembly and a wire protector (U.S. Pat. No. 5,030,220, column 5, lines 39 to 48).
Instead of an aluminum master, Catone described earlier uses of a soft tin alloy for contouring a template in U.S. Pat. No. 5,373,860 (column 2, lines 48 to 50). The patent was directed primarily to bone plate fabrication, particularly for repair of facial bone fractures, and suggested replacing the tin template with a molded impression of the surface area to be repaired; the impression was made by compressing a molding material to the fixation area prior to fabricating an element (column 3, lines 24 to 34). A suggested material was Cranioplast,.RTM. generally used by neurosurgeons to recontour bony defects, and, since the monomer employed in its fabrication is described as toxic to the cardiovascular system and locally irritating to tissues, a film was used as a protective interface during formation of the master (column 4, lines 48 to 59). After the molding material had set, a bone plate or other element was made in two steps by applying force to a compression member to obtain an opposing (positive) impression and using this second contoured impression as a template for the element (column 5, lines 16 to 30). Though tedious, the method was described as having advantages over earlier described bone plate fabrications because manual manipulation during surgery was decreased with more precise contouring (column 2, lines 56 to 65). In U.S. Pat. No. 5,397,361, Clark also made an impression and then used Cranioplast.RTM. material for fabrication of skull plates, but the procedure was not carried out during surgery; instead, the plate was removed and duplicated (column 3, lines 4 to 5 and 19 to 25).
Sanders, et al., disclosed use of a "shape-memory alloy" such as ninitol (a mixture of nickel and titanium) for fabrication of rods for the surgical correction of scoliosis in U.S. Pat. No. 5,290,289 (column 5, lines 5 to 8). Rod shape was changed with heat; rods made from nitinol, for example, reverted to their original shape when heated above their shape transition temperature (id., lines 9 to 12). Entire rods or localized rod portions could be heated with, for example, a radio frequency induction heater, in order to produce selective correctional forces (column 7, lines 43 to 57). Use of templates was not discussed, but the method required rods be deformed in a certain crystalline state to a shape desired for spine correction before use (column 5, lines 51 to 54). Thus, the method involved a number of steps prior to alteration of the rod shape after installation.
Whatever the method of implant element fabrication, a major element of imprecision remains for surgeons repairing or correcting skeletal structures. The placement and arrangement of securing means and engagement of the means with the element are as critical to a successful outcome to the procedure as the contour of the element, and element alignment, positioning and securing are done manually in the operating room. This is particularly true of spinal column corrections and repairs, where various torsional forces in the vertebrae, in the spine fixation system, and between different vertebrae and the system come into play around delicate spinal nerve tissue. Meticulous care must be taken in the positioning of the securing means and shaping of the support members. If the fixation system is not properly fabricated and installed, wire and rod support members can twist and snap and screws or other securing means can fatigue, break or pull out, often resulting not only in a failure of the operation but also in serious medical complications such as spinal cord trauma for the patient.
Successful implantation of spinal support systems is technically complex and difficult, requiring patience and exceptional manual dexterity as well as surgical skill and experience. The procedure is so fraught with possibilities for complications that it is typically employed only when the prognosis without this type of surgical intervention is poor, or casts, braces, traction, and other nonsurgical support and/or corrective treatments have failed.
Moreover, certain spinal support systems are inherently more complicated to install than others. In postero-lateral fusions with pedicle fixation, for example, proper bending and shaping of the connecting rod is a major obstacle to the success of the procedure. The rod is almost always a compound curve which must connect the screws rigidly without placing asymmetrical torque on the screw or the pedicle involved in the fusion. A trial and error method with a variety of instruments has to date been the only available method of shaping the connecting rod.
It would be desirable to have an alternative method for shaping bone support elements that could be used for rapid fabrication in the operating room under sterile conditions. It would be especially desirable to have such a method for shaping spinal support rods and bars that are precisely contoured, including bars that have compound curves.